Photo Credit: AP

All told, the Disneyland measles outbreak infected 147 people in the United States, 131 of them Californians. It was a big outbreak—one of the largest in recent memory, according to the CDC—and greater, still, if you include the infections that spread to Canada. “The Measles Outbreak Is Twice As Big As You Thought,” reads the headline to this Forbes piece, in which Tara Haelle traces the outbreak’s spread north of the U.S.-Canada border. When the article was published on March 12, just over 100 cases, all traceable to two families that visited Disneyland in December, had been reported in the Lanaudière region of Quebec. Today, the number of cases in Quebec linked to the California outbreak sits at 159, exceeding the U.S. tally by 12.

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A spokesperson for Quebec’s Ministère de la Santé et des Services sociaux tells io9 that the outbreak has affected a total of 28 families in the Lanaudière region. The most recent of these 159 cases was diagnosed on March 28th. Health officials in Quebec consider a measles outbreak over when no new cases have been reported for 32 days (that’s not a typo—the all-clear signal is issued ten days earlier in Canada than it is in the U.S.). At least until April 29th, the outbreak will still be regarded as active.

“But that’s Canada,” you may say. “In the United States, the outbreak is over.” Well, yes. But how does one define an outbreak? More importantly, how does one convey the agreed-upon definition of an outbreak to a virus like measles?

California health officials declared the Disneyland measles outbreak over on April 17th because no new cases linked to the outbreak had been reported in the U.S. in 42 days—a timeframe determined by doubling the infection’s 21-day incubation period. In 2000, the United States declared that it had eliminated measles within its borders. (Vaccines! They work!) Elimination, in this context, was defined as the absence of a continuous chain of measles transmission lasting at least 12 months within a defined geographic area. Last year, an expert panel convened by the Centers for Disease Control and Prevention determined that the U.S. had sustained its elimination of endemic measles for the first decade of the new millennium.

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Definitions like these can be useful, but measles—like carbon pollution, or a natural disaster—has no respect for concepts as ecologically meaningless as political boundaries. The vast majority of measles cases in the United States are what epidemiologists call “importations”—someone gets infected with measles overseas, returns to the U.S., and then spreads the disease to others. In January, Dr. Ann Schuchat of the CDC told reporters that while it may never be clear exactly how the Disneyland outbreak started, the assumption is that it was imported.

In an age when international air travel ensures that a measles importation is only ever a plane flight away, what makes Canada’s active outbreak so worrisome isn’t its proximity to the U.S. (the Lenaudière region is only about 100 miles north of the U.S border). Rather, it’s how measles managed to gain a foothold there in the first place. All but two of the 159 cases linked to the Disneyland outbreak have been in people belonging to members of a population that abstains from vaccines “for religious or philosophical reasons”—a population that closely resembles unvaccinated communities across the U.S.

A big reason epidemiologists are worried by big public health emergencies like the Disneyland outbreak is that flareups like these threaten to break the country’s winning-streak against endemic measles. Last year, vaccine-refusal in the United States helped drive measles cases to a 20-year high. The persistence of the Disneyland outbreak in Canada makes it easy to imagine an even worse scenario, one in which the chain of transmission does not stop, but ramify, establishing roots in unvaccinated communities across the U.S., resulting in the outright loss of that country’s elimination status.